The invention relates generally to the recovery of an audio information signal from a variable-area optical record and more particularly to the recovery of such a signal from motion picture film variable-area optical soundtracks.
Variable-area optical soundtracks on motion picture film have been used in substantially their present form since the earliest days of sound-on-film in the cinema. In their earliest form, a single monophonic optical soundtrack was used, the width of the clear area being proportional to the recorded modulation amplitude. Later modifications intended to reduce distortion provided for bilateral and dual bilateral tracks adjacent to each other, carrying the same modulation information and being identical in their pattern. A further modification provides for separately modulating each of the dual bilateral tracks to provide stereophonic reproduction.
Modern commercial film projectors continue to use essentially the same illumination and light sensing arrangements for reading variable-area optical soundtracks as those that were used in the earliest sound projection equipment: a light source and narrow mechanical slit to provide a line source illumination, with a single photocell for detection or with dual photocells in the case of stereophonic dual bilateral tracks. Silicon solar cells have replaced the vacuum tube photocells originally used.
Various techniques introduced since the early 1970s and now in use to improve the quality of optical soundtracks show that the medium is not inherently as deficient as had been supposed in the 1950s and 1960s, when attempts were made to popularize magnetic soundtracks. A useful discussion of the history and potential of optical soundtracks can be found in The Production of Wide-Range, Low-Distortion Optical Sound Tracks Utilizing the Dolby Noise Reduction System, by Ioan Allen in THE JOURNAL OF THE SMPTE, September 1975, Volume 84, pages 720-729. The paper includes a bibliography in the subject area.
A continuing problem in optical soundtrack reproduction is that of achieving a low noise level while providing a wide frequency range. The electrical output level of a solar cell or photocell depends not only on the width of the clear area of the soundtrack but also on the transmissivity of the both the clear and the opaque areas of the soundtrack. Variations in the transmissivity of the clear and opaque areas of the soundtrack are reproduced as noise, transmissivity variations in the clear area being the predominant contributor. The transmissivity of the clear area of the soundtrack is reduced by dirt and scratches; the transmissivity of the opaque area of the soundtrack is increased by pinholes and scratches. The incidence of these transmissivity variations, and hence of print noise, is relatively low in a fresh print, but increases with the number of times a print is projected.
In recent years, improved techniques have reduced other sources of soundtrack noise. On the other hand, the near demise of the projectionist and the increasingly common use of continuous-play platters has increased the rate at which a print wears, and print noise increases, with the number of times the print has been projected. This has meant that print noise has become a problem in first-run motion picture theaters; and that such noise is now the predominant source of noise in motion picture sound systems. Print noise is also less acceptable in second-run theaters, many of which have now installed improved sound systems. The audio noise reduction that is standard in improved motion picture theater sound systems does not deal with the impulsive nature of print noise very effectively.
There have been many prior attempts to eliminate print noise in optical soundtrack reproduction. Many of these prior attempts try to measure the width of the clear area of the soundtrack independently of the transmissivity of the opaque and clear areas of the soundtrack. One early attempt was described in 1944 in U.S. Pat. No. 2,347,084 of Cooney in which the optical soundtrack was repeatedly scanned across its width by a very small spot of light and the light transmitted through the soundtrack was detected by a single photo cell. The output signal from the photo cell was limited to produce an essentially two-level pulse width modulated signal and to reduce the noise caused by clear area transmissivity variations. The pulse-width modulated signal was then integrated to recover the audio signal. This arrangement reduced noise caused by transmissivity variations in the clear area of the soundtrack at the expense of increasing the possibility of transmissivity variations in the opaque areas of the soundtrack causing noise. This was a reasonable trade-off, however, because transmissivity variations in the opaque area are less likely to occur than transmissivity variations in the clear area. A limiter had the additional shortcoming that it could not prevent large variations in the transmissivity of the clear area, such as large specks of dust and scratches, from contributing noise to the output signal.
Cooney also suggested that the image of an illuminated slit could be focussed on the soundtrack in the conventional way and that the resulting illuminated area of the soundtrack could be scanned by a television camera-like scanning device.
U.S. Pat. No. 2,485,829 of Holst et al. shows a scanning arrangement using a threshold detector instead of a limiter to reduce noise caused by transmissivity variations. When the output of the photo cell was below a threshold level, the output of the threshold detector was held in one constant voltage state (e.g., at a low voltage), and when the output of the photo cell was above the threshold level, the output of the threshold detector was held in a different constant voltage state (e.g., at a high voltage). The threshold detector reduced noise because minor transmissivity changes in either the opaque area or the clear area would not change the photo cell output sufficiently to cross the threshold and cause noise generating transitions in the output of the threshold detector. However, large transmissivity variations, such as large specks of dust, scratches, and pinholes, could still contribute noise to the output signal.
In U.S. Pat. No. 4,223,188 of Ray M. Dolby, noise caused by clear area transmissivity variations in the output of a scanned optical soundtrack playback system was reduced by using transitions in the output of the photo cell to trigger a bistable circuit. The bistable circuit started the scan in one state, and was triggered into its other state by the first transition in the output of the photocell caused by an opaque-to-clear boundary in the soundtrack. The bistable circuit was reset to its initial state at a fixed point later in the scan, e.g., at the end of the scan. The audio signal was recovered by integrating the output of the bistable circuit. A development enabled both boundaries of a bilateral soundtrack to contribute to the recovered audio signal by using a transition in the photocell output caused by a clear-to-opaque boundary in the soundtrack to reset the bistable circuit. To reduce the possibility of dirt in the clear area from falsely resetting the bistable, only a clear-to-opaque transition after which the output of the photo cell remained in its "opaque" state for more than a given amount of time, typically 1% to 5% of the scan period, was allowed to reset the bistable circuit. This arrangement was unable to reduce noise caused by transmissivity variations in the opaque area and by contamination of the opaque to clear and clear to opaque boundary regions.
Cooney's scanning system using a television camera detector was combined with the threshold detector of Holst et al. in U.S. Pat. No. 4,124,784 of Johnson et al. Instead of a television camera, Johnson et al. used a 256 element charge-coupled device (CCD) array on to which a magnified image of the slit-illuminated soundtrack was projected. The output of the CCD array was fed into a threshold detector as already described. The threshold detector reduced noise due to transmissivity variations, but variations in the opaque-to-clear and clear-to-opaque boundary regions, and large variations in the transmissivity of both the opaque and clear areas could still contribute noise to the output signal.
All of the scanning devices described above, and other scanning devices not described, although giving improved noise performance compared with a solar cell, suffer from two significant defects: distortion and noise. The inability of simple limiters and threshold detectors to reduce noise due to large transmissivity errors has already been described. Scanning systems also suffer from high distortion because the opaque-to-clear and clear-to-opaque boundaries of the soundtrack are not infinitely sharp; instead, the transmissivity of the soundtrack changes from opaque to clear over a distance of about 200.mu." (5 microns), which is significant even compared with the maximum peak-to-peak amplitude of a unilateral track of about 16 mil. (400 .mu.m), and more significant compared with lower modulation amplitudes.
When the soundtrack is applied to the film, exposure and development parameters which determine the width and shape of the boundary region are chosen so as to minimize distortion when the soundtrack is reproduced by a solar cell. The electrical output of the solar cell represents the total light flux reaching the cell as a result of transmission through the opaque area (negligible), the opaque-to-clear boundary region, the clear-to-opaque boundary region, and the clear area. The proportional contribution of the two boundary regions to the total light flux is different at each point on the cycle of the waveform. It is also frequency dependent: at short wavelengths, the opaque areas between successive cycles tend to grow together across the narrow clear area between them, thus widening the boundary regions.
Known scanning systems employing limiters or threshold detectors to reduce noise produce on each scan an output that is proportional to the effective width of the clear area of the soundtrack at the point at which it is scanned. Because the two boundary regions each have finite width, the measured effective width of the clear area (a) depends on the choice of threshold level or limiting level, (b) is different at different points on the cycle of the waveform, and (c) is frequency dependent. Consequently, a fixed threshold level or limiting level can be chosen that gives low distortion at a given frequency and amplitude, but that fixed threshold level will give significant distortion at other frequencies and levels.
A particularly severe form of distortion afflicts the negative peaks of the recorded waveform. On negative peaks, the clear area is so narrow that the two boundary regions of the soundtrack overlap and the clear area becomes partially opaque. This effect is exacerbated by the tendency of opaque areas to grow together when the clear area between them is narrow. Thus, the soundtrack has no area that a scanning system with a fixed threshold or with limiting would interpret as being clear. This results in clipped negative peaks in the audio output of the scanning system. Because the width of the bias line (the bias line is the nominally clear area that always exists between the two halves of a bilateral track) is varied according to the amplitude of the waveform recorded on the soundtrack to minimize the amount of clear area in the soundtrack, negative peak clipping can occur with signals of all levels.